Copilot CLI never prunes ~/.copilot/session-state, causing EMFILE / file-descriptor exhaustion (crashes VS Code Copilot Chat)

### Summary

The Copilot CLI creates a `~/.copilot/session-state//` folder for **every** session — including background automations, heartbeats, and memory-consolidation runs — and **never prunes them**. On a heavily-used machine these accumulate into the thousands. A Copilot component then bulk-opens `session-state/*/workspace.yaml`, exhausting the process's file-descriptor table (libuv's ~8189 limit on Windows) and throwing `EMFILE: too many open files`. In VS Code this makes Copilot Chat hang on "working" and crash after a few turns.

This is the root cause behind microsoft/vscode#313799 — that's where it *manifests*, but the fix belongs here.

### Evidence (Windows, measured with Sysinternals `handle.exe`)

- `~/.copilot/session-state` contained **10,313 `workspace.yaml` files**, accumulated since 2026-02-18, never pruned.
- The Copilot `node.mojom.NodeService` utility process peaked at **~8,788 open handles** — right at the libuv fd ceiling — then dropped back to ~580 after the burst (which is why point-in-time snapshots look normal).
- In a single mid-spike handle dump, **3,256 of 3,352 (97%) File-handle paths were `session-state\...\workspace.yaml`**.
- Reducing the folder count to ~67 **immediately** resolved the EMFILE crashes.

### Two underlying issues

1. **No retention/cleanup (primary):** session-state folders grow unbounded — no age or count cap, no garbage collection.
2. **Unbounded fan-out open:** the code that reads `workspace.yaml` across sessions opens them without bounding concurrent file descriptors or closing promptly, so a large directory exhausts the fd table instead of degrading gracefully.

### Suggested fixes

- Add automatic retention (e.g., prune session-state older than N days, or cap to the M most-recent) on CLI startup and/or via a background task.
- Bound concurrency and ensure prompt handle closure when enumerating session metadata.
- Optionally consolidate session metadata into a single index/DB rather than one folder + file per session.

### Workaround

Periodically prune `~/.copilot/session-state` (e.g., a scheduled task that deletes folders older than ~14 days).

### Environment

Windows; GitHub Copilot CLI + VS Code Copilot Chat (stable).

Bắt đầu bằng cách xác định mã CLI tạo và đọc ~/.copilot/session-state, sau đó lần theo các điểm vào của quá trình khởi động hoặc tác vụ nền, cùng với việc liệt kê workspace.yaml. Tái hiện trường hợp phiên lớn trên Windows và xác minh rằng cơ chế lưu giữ xóa trạng thái cũ, các lần đọc tệp được đóng kịp thời và CLI không còn đạt đến EMFILE với khối lượng công việc đã được báo cáo.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.